This invention relates to electrical appliance control and particularly concerns remote appliance control effected by way of digital control signals transmitted on a power main.
U.K. Pat. No. 1592971 discloses such a remote control system for electrical appliances in which receivers control the supply of power to the appliances, the receivers themselves being controlled by digital instruction signals conveyed by the power main. The instruction signals, generated by a transmitter coupled to the main, may include, in some cases, an address to identify a particular receiver to be controlled.
Preferably, each instruction or message comprises a plurality of binary bits transmitted during selected time slots relative to the zero-crossing points of the main. Specifically one set of transmission time slots occurs close to the zero-crossing points of one phase of the main. In order that the instruction or message may be coupled to other phases of the main, other sets of transmission time slots are provided close to the zerocrossing points of the other phase or phases of the main. By "close" to a zero-crossing point, we mean that at least a portion of a time slot is within one eighth (preferably one tenth) of one half cycle from a zero crossing.
In the above-mentioned Patent, each bit is transmitted as the presence or absence of a high frequency (e.g. 120 KHZ) signal during the time slot. Moreover, the bits of the message are transmitted during alternate half-cycles and the inverse of those bits is transmitted during the intermediate half-cycles. Particularly, a transmission time consists of a sequence of time intervals with sections or words of a message transmitted in alternate time intervals, the intermediate intervals being long enough (preferably equal the alternate intervals) to to contain a repetition of the word. Preferably each word is also transmitted in the intermediate time intervals so that a complete message is transmitted twice.
A number of problems can occur when using such a system to control the use of energy within buildings such as factories, offices, schools and hospitals. The most common problem is low signal level at the receivers or slaves. A typical installation might have a transmitter operating on a branch circuit of a 3-phase, 60 HZ, power distribution circuit, and it is required to address several receivers located on many branch circuits on all 3 phases throughout the building. Signal attenuation occurs due to range and to shunting of the signal by loads. However, the greatest loss is usually due to poor phase coupling. Phase coupling can be improved by the use of passive components such as capacitors, or transformers; however, the net output power of the transmitter is not increased by these methods.
Other problems arise owing to the diverse types of power distribution used in such buildings, (this applies mainly in the U.S.A.). For example, two systems in common use are:
(i) the 3 phase, 4 wire 120 V/208 V system; and PA1 (ii) the 3 phase, 4 wire 277 V/480 V system.
System (i) is used normally where motors form a substantial part of the load. System (ii) is used mainly for fluorescent lighting.
System (i) can provide 3-phase, 120 V power between each of the transformer outputs and neutral. It will also supply 208 V A.C. for loads connected between any pair of transformer outputs. The zero voltage crossings of the 208 V connections are 30 deg. delayed from those of the respective 120 V connections, (1.38 m. secs. for a 60 HZ system).
The pulses from a transmitter located on a 120 V branch circuit would not be received by a receiver with its input circuit across one of the 208 V connections because the signal time slots would not be appropriately timed. A similar problem arises for the 277 V/480 V system. Normally in a building the 277 system would be distributed and the 120 V derived where locally required by step down transformers from one phase of the 277 V. This usually introduces a 30 deg. phase shift between the local 100 V systems and the 277 V system.
In data transmission systems, it is known to provide a repeater in series with the data transmission pattern to attempt to resolve problems of low signal level.
Moreover in a power line communication system, it is known from U.K. Specifications No. 1535834 to provide a repeater to by-pass distribution transformers at which severe signal loss can occur. Such a system uses frequency shift keying and the frequencies are changed by the repeaters. The repeaters also change the form of the signal. The system appears therefore to be inoperable in the absence of repeaters as the final receivers cannot respond to the signal as originally injected onto the main. Such a system does not therefore provide any convenient answer for an electrical appliance control system as mentioned above in connection with U.K. Pat. No. 1592971.